Within industry, indicators are often used to convey the status of a signal or information that is being monitored. Indicators are often utilized to communicate the presence of a particular signal or state. However, in order for the absence of a signal to be indicated with any certainty, additional verification is required.
When attempting to indicate the absence of a signal with any certainty, positive indication is required. This is particularly important when using an electronic indicator, such as a light emitting diode. For instance, an indicator could be used to indicate the presence of a signal such as power, voltage, communication, etc. by illuminating when the signal is present. However, if the indicator is not illuminated assuming that the signal is not present, although true in some instances, would not be accurate all of the time. There are several possible scenarios to consider: the indicator may have failed or reached the end of its life; one or more components in the circuitry comprising or controlling the indicator may have failed; or the connection between the signal and the indicator may be detached or severed. Therefore, in order to verify the validity of a signal with any degree of confidence and to positively indicate that absence, continuity between the signal source and indicator must be established and confirmed.
One application for this type of positive indication of the absence of a signal deals with voltage detection. Prior to performing de-energized work on electrical equipment, workers are required to verify that the equipment is in an electrically safe state. Until proven otherwise, one must assume that the equipment is energized and take all necessary precautions, including utilizing appropriate personal protective equipment (PPE). One part of the verification of an electrically safe work condition involves a test for the absence of voltage. This test is performed by a trained and qualified electrical worker using an adequately rated voltage tester, usually a portable voltmeter or multimeter. The electrical worker first tests his meter on a known, energized source to ensure it is working properly. He then verifies that voltage is absent in the electrical equipment by metering phase-to-phase and phase-to-ground. Finally, he re-tests his meter on a known, energized source to ensure it is still functioning properly and wasn't damaged during the test. Although voltage verification is an NFPA 70E requirement and considered a best practice, the test itself still presents a hazard because workers are exposed to energized circuits and conductors when using the voltage tester during the live portions of the test and it relies on humans to follow a process, verifying the meter functionality before and after use. Thus, a permanently installed device that is able to detect the presence and verify the absence of primary (single- or multi-phase AC or DC) voltage, provide a means of testing itself, and positively indicate the status of voltage in a particular electrical compartment would be useful for this type of application.